His criminal father, hands down, he was punishing (paying back) good law abiding Americans for the authorities taking his criminal father away from him.  Criminal mind genes passed down may have played a role in allowing him to get this whole thing on line without drawing attention.  Some people are born with really twisted brains that no parent, psychologist, police, or institution can observe or save.  Most criminals are driven by environment to go "bad" they might be reformed given the chance.  But people like Dahlmer, Gaci, Manson, Hitler, Paddock etc. etc. can't be fixed.

from an article:

Mental illness inherited from father
Investigators are also looking at the twisted killer’s life, probing whether he inherited his criminal father’s mental illness and ruthless passion for violence.

Paddock was the son of Benjamin Hoskins Paddock, a serial bank robber who ended up on the FBI Most Wanted list back in 1969 when he escaped from federal prison in Texas while serving a 20 year sentence.

The FBI kept him on the list for the next eight years, and he was eventually found one year after he was removed from the list in 1978 while outside an Oregon Bingo hall.

The agency said that the fugitive had been ‘diagnosed as psychopathic’ and also had possible ‘suicidal tendencies’.

He father died a few years ago.

Paddock's brother Eric has said the pair never knew their father.

Attempting to find an explanation, Eric added: "Something happened, he snapped or something."